Formation of positive ions and other primary species in the oxidation of sulphides by hydroxyl radicals

Abstract

The oxidation of simple aliphatic sulphides [MeSMe, EtSEt, (CH,),S] by hydroxyl radicals occurs via a complex reaction mechanism. The first step is addition of the OH* to sulphur to form R2SOH radicals. At low sulphide concentrations ( < ~O -* M ) R,SOH rapidly eliminates H 2 0 to form a RSR(-H)* radical which may be described by the mesomeric forms -CH-S-and -CH=S-.This radical is ultimately also formed at higher sulphide concentrations but via a different pathway. R2SOH increasingly reacts with another R2S molecule to form a short lived (R,S),OH* radical complex which dissociates to (R,S),+ and OH-. The (R,S),+ complex ion seems to be relatively stable and decays essentially via equilibration to the molecular cation R2S+. This ion in its reaction with the solvent, OHions, and through a bimolecular process with another R2S+ cation is effectively deprotonated to form the RSR( -H). radical. The reaction route a t high concentration includes the formation of transient species with oxidizing properties; Fe(CN)64is rapidly oxidized by (R,S),+ [and possibly (R,S),OH*].The RSR(-H).radical partially disproportionates to negative and long lived (>I ms) positive ions. The stable oxidation product, sulphoxide, has been identified.

Photosensitized oxidation of methionine derivatives. Laser flash photolysis studies

“…This lack of BP d-formation is likely due to the fast proton transfer within a solvent cage, yielding the BP radical. This proton is presumably donated by the quencher molecule, yielding the relatively stable C-centered radical [18], which could not be observed in the flash photolysis due to its inconvenient absorption overlap with the BP ground state. Despite of those difficulties, the supporting evidence for the proposed reaction mechanism came from the steady-state irradiation experiments, where the product of recombination of the BP ketyl radical and a C-centered radical of N-Ac-Met-OCH 3 (formed upon deprotonation of the initially formed S-centered radical cation) was detected and identified by means of HPLC-MS analysis.…”

Odd‐Electron Bonds

“…Above all, much of the late Dieter Asmus' seminalw ork [4][5][6][7][8][9][10][11][12][13] has apparently been forgotten, [72] but also the pioneering experimental studies of Meot-Ner, [19] Pauling [3] and Hiberty's [29] rationalization of oddelectron bond energiesi nt erms of valence-bond theory and Bock [34] and Nelsen's [35] fundamental studies on three-electron p-bonds are apparently no longerp resenti no ur general picture of bondingt heory.T his short review attempts to remind us of these classical examples of physicalo rganic chemistry, which taken on an even more importantr ole as the importance of radicals and radical ions is increasingly recognized in both biology and photovoltaics and whose importance in transition-metal chemistry has recently been reviewed. [73] Computational Section All calculations used the Gaussian 09 series of programs.…”

“…Theoretical interest in odd-electron bonding was strengthened by Asmus' observation of intramolecular SÀS three-electron bonds in solution. [4] This was followed by similar studies on S-halogen three-electron bonds [5][6][7][8][9] and acyclic SÀS three-electron bonded radical cations. [10,11] Furthert hree-electron bonds of sulfur with nitrogen, oxygen [12] and phosphorus [13] and between two nitrogen [14,15] and two iodine [16] centers [17] werea lso reported.…”

Free radical oxidation of organic disulfides

“…This conclusion is also in agreement with our previous finding that the complexed radical cation (RzS)2.+ is rather stable and reactions essentially occurred through the uncomplexed R2S.+. 24 Further support for this mechanism arises from the corre-5.3 X loW3, 3.6 X sponding results from N20 saturated solutions containing di-tert-butyl thioether M). In contrast to the other thioethers no complexed radical cations are formed (probably due to steric reasons)24 and the monomer radical cation is immediately available for the reaction M) and diethyl disulfide…”